ToDays() is a conversion function. This converts TimeSpan to days.
Lets say you want to know the no. of days between two dates 26/5/2018 and 26/5/2011. In such case this function can help you. In the expression of ToDays(), have the below -
ToDays(MakeDate(26,5,2018) - MakeDate(29,5,2011)) will give you 2554 days.